One known method for generating hydrogen using, as material gas, hydrocarbon-based fuel such as city gas (natural gas) and LPG involves a steam reforming reaction in the presence of a catalyst. In the steam reforming reaction, the material gas and steam are made to react with each other on a reforming catalyst having a high temperature of e.g., 600° C. to 700° C. thereby to yield reformed gas composed of a mixture of hydrogen, methane, carbon monoxide, carbon dioxide, steam and others.
When utilizing the hydrogen of the reformed gas in a fuel cell, carbon monoxide, which causes poisoning of the fuel cell, has to be eliminated from the reformed gas. Therefore, the amount of carbon monoxide is reduced to 10 ppm or less, for example, through a shift reaction in the presence of a shift reaction catalyst or a selective oxidation reaction that is carried out subsequently to mixing with oxygen, utilizing a selective oxidation catalyst. To properly carry out such reactions, hydrogen generators are constructed to select a catalyst type and establish a catalyst temperature and a reaction gas flow condition in which the catalyst can exhibit its strongest reactivity. (see Patent Literature 1 and Patent Literature 2).
Incidentally, exposure to water and carbon deposition are well known as the causes of degradation of the catalysts used in the hydrogen generators. Although steam is necessary for the steam reforming reaction, if steam is supplied to the catalyst when the temperature of the catalyst is not sufficiently high, the steam will be cooled and condensed by the catalyst and the structural elements located about the periphery of the catalyst. The water produced by the steam condensation adheres to the surface of the catalyst and such adhering water infiltrates into the catalyst. The water adhering to the surface of the catalyst causes a change in the composition of the catalyst, adversely affecting the catalytic performance. Meanwhile, the water, which has infiltrated into the catalyst, generates a great power within the catalyst owing to its rapid volumetric expansion when it evaporates within the catalyst during the period of catalyst temperature rise, which would lead to catalyst destruction.
The other cause of catalyst degradation, that is, carbon deposition could occur on the catalyst, inside the catalyst and on the structural elements of the hydrogen generator surrounding the catalyst, if the material gas, which is a carbon-containing hydrocarbon substance, is supplied alone when the catalyst and the structural elements are at high temperatures. If carbon deposition occurs, the active site of the catalyst will be covered with carbon, resulting in a decrease in catalytic activity. Carbon deposition inside the catalyst may cause a decrease in the crush strength of the catalyst itself, which leads to pulverization of the catalyst. Also, carbon deposition may cause blockage in the passage for the gas flowing between the catalyst and the structural element, entailing flow deviation that adversely affects the performance of the catalyst layer on the whole.
One proposal to avoid such undesirable situations is disclosed in the aforesaid Patent Literature 1 according to which the temperature of the steam generator for generating steam is detected and steam is supplied to the reformer after confirming that the steam generator is ready to supply steam. In this arrangement, the catalyst is heated by the burner in the reformer until the steam generator reaches a state in which it can supply steam, but if it is determined that the temperature of the catalyst has been excessively raised to such an extent that carbon deposition occurs, the supply of the fuel gas to the burner is stopped to stop the heating of the catalyst with the burner. This prevents carbon deposition in the catalyst to ensure the durability of the catalyst.
There has been known an alternative operation method for a hydrogen-containing gas generator (e.g., Patent Literature 3). According to this method, in response to a start-up command, a heating process starts to heat the reformer and the shift converter with heaters. After the temperature of the reformer and the shift converter has risen to a level at which carbon deposition due to the thermal decomposition of the desulfurized raw fuel gas as well as steam condensation can be prevented, a steam substitution process is performed to feed steam to the reformer. Sequentially, a processing-object-gas supplying process is performed to feed the desulfurized raw fuel gas and steam to the reformer, upon the reformer temperature reaching a level at which reforming becomes possible. In the operation method for a hydrogen-containing gas generator disclosed in Patent Literature 3, upon start-up of the hydrogen-containing gas generator, the heating process starts in which the reformer is heated with a combustor and the shift converter is heated with a start-up heater.
An alternative reformer for a fuel cell has been proposed (see e.g., Patent Literature 4). This reformer for a fuel cell has a first heater for heating the reforming catalyst and the carbon monoxide removing catalyst through combustion of fuel for use in heating. In this reformer, an electric heater for heating the carbon monoxide removing catalyst is provided in the carbon monoxide removing section and this electric heater is turned ON when supplying the first heater with the fuel for heating. According to the reformer for a fuel cell disclosed in Patent Literature 4, the electric heater is turned ON when supplying the fuel to the first heater, whereby the carbon monoxide removing catalyst is not only heated by the first heater but also heated from the inside thereof by means of the electric heater.